The Treasury Department released figures on Monday showing the federal budget deficit widened by 17 percent in the 2018 fiscal year, to $779 billion. That’s an unusual jump for a year in which unemployment hit a five-decade low and the economy experienced a significant economic expansion. But the increase demonstrates that the tax cuts President Trump signed into law late last year have reduced federal revenues considerably, even against the backdrop of a booming economy.

Some conservatives don’t see the rising deficit numbers that way. They note that the Treasury reported that federal revenues rose by 0.4 percent from the 2017 fiscal year to the 2018 fiscal year, and view that as a sign that the tax cuts are “paying for themselves,” as Republicans and Mr. Trump promised.

That’s not the case. ...

The issue here is not whether the government spends too much money, or whether tax cuts have buttressed economic growth, or even whether it’s advisable to run such high deficits in flush economic times.

The issue instead is: Have the corporate and individual tax cuts that went into effect in January generated so much additional growth that tax revenues are as high, or higher, today than they would have been if the tax cuts never passed? That’s how all scorekeepers — be they independent congressional staff members or researchers from think tanks that lean liberal or conservative — assess the “pay for themselves” question. ...

By the Treasury’s numbers, total revenues grew 0.4 percent from the 2017 fiscal year to the 2018 fiscal year. That’s weak, historically speaking, for an economy growing as fast as it is; in the 2015 fiscal year, when growth was comparable to what it is today, revenues grew 7.5 percent from the previous year.

The weak growth brought revenue as a share of gross domestic product down, something that typically happens in — or around — a recession, not deep into a robust expansion that the Fed has described as a “particularly bright” moment.

But revenue is definitely growing after the tax cuts, right?

Well, no.

The fiscal year runs from the start of October to the end of September. The tax cuts mostly took effect in January 2018. That means three months of the 2018 fiscal year included a period without the tax cuts in place. If you look only at the nine months after the cuts took effect, you’ll see that revenue is ever so slightly down, year over year: From January through September 2017, revenues were $2.57 trillion. For the same period in 2018, they were $2.56 trillion. Which is to say, they’re down by $10 billion, in a direct comparison after the tax cuts started.